Confirming PSU spec for steppers.

[JAZZCNC]

Yeah, he likes to give me expensive advice. I'm still trying to work out how to safely situate this machine after he got me to check the rafters Keep an eye out for the upcoming threads on building waterproof, soundproof and giant-maneating-spider-proof low profile enclosures with built in dehumidifiers....

You forgot " That are electricly safe". . :)

[Rogue]

You forgot " That are electricly safe". . :)

Actually I'm counting on the sparks, arcing and random live surfaces to keep the giant-maneating-spiders away if I end up having to use the basement...

[irving2008]

Yeah, he likes to give me expensive advice. I'm still trying to work out how to safely situate this machine after he got me to check the rafters Keep an eye out for the upcoming threads on building waterproof, soundproof and giant-maneating-spider-proof low profile enclosures with built in dehumidifiers....

Not that I mind in the least of course. I'd rather have the advice to weigh up than not have it and blunder on blindly.

I'm intrigued though. What kind of thing would you be doing to benefit from the price difference?

The cheap capacitors are a bit marginal on ripple current for three but as Jazz/Jonathan says they'll do the job for the level of use you'll put them to esp if you use four of them. I did qualify my statement with the phrase... "or whatever you can get cheap on eBay :)" and they were examples. I have 2 x 7500uF 400V capacitors on my 68v supply, I paid £5 each on eBay - they list at £75 each, used in hi-quality 500W+ sound systems - the D-class output stage of a high-power switching amp looks remarkably similar to a stepper drive :). The only thing I don't like about the ones you've chosen are the solder terminations; its a personal preference but I like meaty screw terminals.


On other points...

Bleed resistors are there for safety. You might never power the supply on without the steppers connected but I can't know that and I would never advise building a hi-voltage (>48v) supply without them. Leaving them out isn't best practice (it ranks with wiring e-Stops with 230v and having plugs on the 'live' side in my book) and I've been caught out by that before. For the record, I've been in electronics manufacturing on and off for over 30y and have seen most levels of stupidity :)

As regards the heatsink Jonathan, of course the case doesnt get to 150degC, its what the junction mustn't go above (actually 125degC for your device). If you look at the datasheet for the rectifier you used its rated at 35A @ 55degC case temp. At 35A its dissipating 35 x 1.1 x 2 = 77W so given the limiting junction temp is 125degC the internal thermal resistance must be (125-55)/77 = 0.9degC/W therefore case to ambient of 25degC must be 0.4degC/W to run with no heatsink (which is what the datasheet seems to suggest, though I find that very low, but it is a metal case so go figure). At 12A (which I accept is a worst case with all three motors at full chat, relatively rare) your case temp would be 35degC. On the face of it thats a better rectifier for this purpose. The one I suggested has an epoxy case so clearly needs the heatsink as detailed in its datasheet.

[JAZZCNC]

Bleed resistors are there for safety. You might never power the supply on without the steppers connected but I can't know that and I would never advise building a hi-voltage (>48v) supply without them. Leaving them out isn't best practice (it ranks with wiring e-Stops with 230v and having plugs on the 'live' side in my book) and I've been caught out by that before. For the record, I've been in electronics manufacturing on and off for over 30y and have seen most levels of stupidity :)

Couldn't agree more and here's an example of why.!
I've just acquired an old cnc lathe out of a school and been ripping it's guts out has it runs a bespoke control/software and I want to run Mach3.
This thing has 3 boards Control boards, PSU board and spindle board. I'm keeping the PSU and Spindle so after pulling the control board out I was messing with the spindle board trying to work out what wire goes where when I got a pretty wicked kick I wasn't expecting seen has it wasn't plugged into mains.?? . . . .Yep it's got toroidal PSU with no Cap drain.

Not enough to hurt me but woke me up good and that was only small PSU running at 36V ish with relatively small caps.!!

SO I completely agree with Irving and always build them into my PSU's..!! . . Now the only thing I don't like about Irvings way is that the resistors are permanently draining while the machines running, this creates heat while running so warms the control box and therefore the drives etc.
I'm a bit Anal with my control box and build them with relays and full safety using 24V and control lots of things with relays, one of these is a NC relay which kicks in and out a resistor so it only drains when off.!! . . No such thing has being too safe IMO.

[Rogue]

For the record, I've been in electronics manufacturing on and off for over 30y and have seen most levels of stupidity :)

And I'm grateful for the chance to be able to draw on that experience, along with the experience and knowledge that Jazz and Jonathan have brought to the discussion.

Fuses - now that never occured to me but sounds like a rather good idea...

[Jonathan]

You might never power the supply on without the steppers connected

To be fair it's a good idea to power the supply on without the steppers connected when you've first built it to check the voltage is correct, so yes have the bleed resistor.

As regards the heatsink Jonathan, of course the case doesnt get to 150degC, its what the junction mustn't go above (actually 125degC for your device).

I should have clarified I've used both on different power supplies. It's 150°C for the first one I linked to and 125°C for the other.

I just put an ammeter on my 500VA PSU and ran a few programs on the router. With 4 motors maximum current was 2.66A and at standstill 0.72A. Just going by eye (not great I know, should use my oscilloscope) the mean current looked to be about 1.9A. The drivers are set to 50% current at standstill, so since the power supply is outputting 68V we expect the current current when stationary for 4 motors to be 4*0.5*2.73*4.2/68*2=0.674A, so the measurement is fairly close.

So lets be very safe and calculate it with the peak current since we know if the temperature is fine at that current (2.66A) it must be fine for the lower current when running. The 8A rectifier I used is 2.2°C/W junction to case and forward voltage is 1V, so 2.66*1=2.66W to dissipate. (150-25)/2.2=68°C/W which is a large value, hence why it works with no heatsink. Also the datasheet specifies up to 3.2A with no heatsink, so clearly we're fine especially as in reality the mean current for 3 motors is less than 2.66A.

Edit: Just tried measuring using the oscilloscope with a shunt resistor and running one motor at the speed it draws the highest current I get 1.58A mean, so 4.75A for 3 motors which implies if I get all 4 motors running at their peak, it will draw 430W from the 500VA transformer.

[Rogue]

You'll also need a bleed resistor across each capacitor to discharge it when the power is turned off (for safety). The energy stored is 0.5CV^2 joules. For each capacitor spec'd above, the energy is 0.5 * .005 * 70^2 = 6joules. A watt is a joule per second, so for a 5 second discharge you need to dissipate 6/5 = 1.2W so we're going to need 2W rated resistors. Assuming the resistor dissipates the same 1.2W when the power supply is turned on the value will be R = v^2/W = 70^2/1.2 = 4000ohm. I'd use 3900ohm (3k9) @ 2W wire ended and solder them directly to the ring tags on the capacitor: Buy Through Hole Fixed Resistors Carbon Resistor, 2W ,5%, 3k9 RS RS-Carbon-3k9-5%-2W online from RS for next day delivery. (pack of 10 @ 72p)

After a cup of tea and a bit of a read, I have a quick question in relation to 0.5CV^2. Where you give 0.5 * 0.005 * 70^2 = 6, I end up with 12.2. In fact using the time honoured technique of "randomly changing numbers until they worked", I only got 6j by changing the 70v to 16v?

Trying to follow your calculation and sticking with the 5 second discharge, I ended up with this:
0.5 * 0.005 * 70^2 = 12.25j
12.25 / 5 = 2.45W
70^2/2.45 = 2000ohm, so I would need a 2k ohm @ 3W resistor to discharge over 5 seconds.

I'm rather hoping you made a typo because the alternative is that I'm more stupid than I thought!

[irving2008]

After a cup of tea and a bit of a read, I have a quick question in relation to 0.5CV^2. Where you give 0.5 * 0.005 * 70^2 = 6, I end up with 12.2. In fact using the time honoured technique of "randomly changing numbers until they worked", I only got 6j by changing the 70v to 16v?

Trying to follow your calculation and sticking with the 5 second discharge, I ended up with this:
0.5 * 0.005 * 70^2 = 12.25j
12.25 / 5 = 2.45W
70^2/2.45 = 2000ohm, so I would need a 2k ohm @ 3W resistor to discharge over 5 seconds.

I'm rather hoping you made a typo because the alternative is that I'm more stupid than I thought!

Errrmmm... looks like I made a boo boo... might have multiplied by 0.5 twice in error... your calculation is correct... now I'll go and edit my post..

BTW 2k2 @ 3w will be fine

[irving2008]

Again, its good practice to have 'voltage present' indicators, especially if you have a secondary fuse. (plus I like lots of lights :) ).

For the primary side a panel mounted neon indicator wired directly across the transformer primary.

For the secondary side a 5mm LED mounted in a panel mount with a 3k9 2W resistor wired across the output.

[Rogue]

Then I shall take off my dunce's cap and come out of the corner. For now, at least